The present invention is related to disc drives. In particular, the present invention is related to servo systems.
In a computer disc drive, data is stored on a computer disc in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is accomplished utilizing prerecorded servo information either on a dedicated servo disc or on angularly spaced sectors that are interspersed among the data on a disc. During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES), which provides an indication of the position error of the head away from the track center. The PES is then converted into an actuator control signal, which is fed back to control an actuator that positions the head.
As the areal density of magnetic disc drives increases, so does the need for more precise position control when track following, especially in the presence of external shock and vibrations. In order to desensitize disc drives to translational vibrations, a balanced mechanical actuator has been typically used. However, since the actuator must pivot freely to access the data, the effects of rotational vibration about the axis parallel to the pivot axis can be considerable.
To eliminate such errors, a disc drive may introduce cancellation signals into the servo loop that are designed to cancel the vibration-induced runout. To generate an appropriate cancellation signal, the disc drive must accurately measure the vibration of the disc drive using an internal accelerometer.
Since the positioning of the head requires a great deal of accuracy, it is preferred that the sensors be as accurate as possible. In addition, since space within the disc drive is limited, it is preferred that the sensor be as small as possible so that it does not take up valuable printed circuit board space. One type of sensor that is small and accurate is known as a Micro Electro-Mechanical System (MEMS). However, these sensors generate data at a rate that is incompatible with the lower data rate of the servo loop. Moreover, the rate at which these sensors produce data is generally fixed because the data rate is a by-product of the sensing characteristics of the sensor. Thus, the data rate of a MEMS sensor cannot be changed much once the other performance characteristics of the sensor have been chosen. As such, a sensor is needed that can be used with a servo system to generate a cancellation signal while providing the size and accuracy benefits of a MEMS sensor.
A method and apparatus are provided for positioning a head based on a control signal and a cancellation signal. In the method and apparatus, a sensor located on a printed circuit board assembly is capable of generating sensor data samples at a sensor data rate that is different from a servo data rate. A data enlarger inserts additional data samples between the sensor data samples to produce enlarged data that is filtered to produce filtered data, which is decimated to form decimated sensor data at the servo data rate. A cancellation component converts the decimated sensor data into a cancellation signal that may be combined with a control signal to control the actuator.
A sensor chip is also provided that includes a sensing element and a sigma-delta circuit that cooperate to generate sensor samples at a sensor data rate. The sensor chip also includes a buffer that is connected to the sigma-delta circuit and that is capable of storing sensor samples from the sigma-delta circuit at the sensor data rate. The buffer is also able to provide stored sensor samples at a rate that is higher than the sensor data rate. In one embodiment, the sensor chip is able to provide the stored sensor samples to an off-chip component at a rate set by the off-chip component.